A fuel vapor treatment system is used for preventing fuel vapor produced in a fuel tank from being dissipated into the atmosphere and introduces the fuel vapor in the fuel tank into a canister accommodating an adsorbent to adsorb the fuel vapor temporarily by the adsorbent. The fuel vapor adsorbed by the adsorbent is desorbed by negative pressure produced in an intake pike when an internal combustion engine is operated and is purged into the intake pipe of the internal combustion engine through a purge passage. When the fuel vapor is desorbed from the adsorbent in this manner, the adsorbing capacity of the adsorbent is recovered.
When the fuel vapor is purged, the flow rate of an air-fuel mixture containing the fuel vapor is adjusted by a purge control valve provided in the purge passage. However, to adjust the amount of fuel vapor actually purged into the intake pipe to a suitable air-fuel ratio by the purge control valve, it is important to measure the concentration of the fuel vapor in the air-fuel mixture flowing through the purge passage with high accuracy.
In the related art, for example, as disclosed in JP-5-18326A, mass flowmeters are set in the purge passage and in an atmosphere passage branched from the purge passage. The concentration of the fuel vapor in the air-fuel mixture supplied to the purge passage of the internal combustion engine from the purge passage is detected on the basis of the output values of the two mass flowmeters.
However, in this system, since the mass flowmeter is set in the purge passage, the concentration of the fuel vapor cannot be detected unless the air-fuel mixture containing the fuel vapor is purged and is flowed through the purge passage. For this reason, to reflect the detected concentration of the fuel vapor to an air-fuel ratio control, it is necessary to finish detecting the concentration of the fuel vapor before the purged fuel vapor reaches an injector. It is necessary to correct a command value of the amount of injection of fuel to be injected from the injector by the use of the concentration of the fuel vapor.
However, in the case that the volume of an intake pipe is small or the velocity of flow of intake air is fast, the time required for the purged fuel vapor to reach the injector is shorter than the time required to finish measuring the concentration of the fuel vapor. There are cases where it is not possible to reflect the measured concentration of the fuel vapor to the air-fuel ratio control from the start of purge. Thus, this results in limiting an engine structure such as the layout of piping and an operating range where purge is started.
In view of these points, the present applicant has invented and applied a system capable of measuring the concentration of fuel vapor contained in an air-fuel mixture irrespective of purging the air-fuel mixture containing the fuel vapor (refer to U.S. Pat. No. 6,971,375 B2). This system has a pump provided in a measurement passage having a restrictor and can produce a gas flow in the measurement passage and has a switching valve for switching gas flowing in this measurement passage to either air in the atmosphere or an air-fuel mixture containing fuel vapor. The system has a differential pressure sensor for measuring a differential pressure developed across the restrictor when a gas flow is produced in the measurement passage and measures a differential pressure when the gas flow is air and a differential pressure when the gas flow is an air-fuel mixture containing fuel vapor.
Here, as the concentration of fuel vapor contained in the air-fuel mixture becomes larger, the density of the air-fuel mixture becomes larger, so a differential pressure across the restrictor becomes larger. A differential pressure ratio between a differential pressure when the gas flow is air and a differential pressure when the gas flow is air-fuel mixture is nearly proportional to the concentration of fuel vapor. Thus, the concentration of fuel vapor can be found from the differential pressure ratio.
In the above-mentioned system, the operation of switching gas flowing through the measurement passage to air and air-fuel mixture by a switching valve is necessary for measuring the concentration of fuel vapor. For this reason, when the switching valve cannot perform the switching operation normally, it is important to detect an abnormality in the switching valve quickly.